The invention relates to a low-pressure sodium discharge lamp comprising:
a discharge vessel which is sealed in a vacuumtight manner and has a filling comprising sodium and rare gas; PA1 electrodes arranged in the discharge vessel and each connected to at least one respective current conductor which issues to the exterior through a respective pinched seal, wherein the current conductors each have a first coating of borate glass which extends from within the relevant pinched seal to inside the discharge vessel, and in contact with said coating a second, lime glass coating which extends to outside the discharge vessel; PA1 an evacuated outer bulb which is provided with an IR reflection filter and which surrounds the discharge vessel.
Such a low-pressure sodium discharge lamp is known from U.S. Pat. No. 4,783,612.
In the known lamp, the coatings have a butt joint which lies inside the relevant pinched seal. The first coating is resistant to sodium. This coating protects the current conductors against electric contact with liquid or solid sodium which could deposit against the pinched seal. The discharge arc could apply itself to the sodium if the coating were absent, which would lead to violent reactions and damage to the current conductor and the pinched seal. The second coating is thicker than the first one and absorbs forces which result from the difference between the coefficients of thermal expansion of the discharge vessel and the current conductors.
The construction of the known lamp was found to be reliable in the case of operation at mains frequency. With high-frequency operation, however, cracks arise in the pinched seal after a few thousand hours already, leading to lamp leaks and thus to the end of lamp life.